The invention is generally related to risers for floating offshore oil and gas production structures and more particularly to air-can tensioning devices for the risers.
In the production of oil and gas at offshore locations, it is necessary to support the risers used in production and drilling operations. Air-can tensioning devices are commonly used to provide such support. The air-cans use buoyant forces to support and over tension the risers that extend from the structure down to the sea floor. Referring now to FIGS. 1 and 2, a conventional air-can 7 is seen located around stem 32. Lower sections have open ports 30a-30c and are pressurized via air-lines 5a-5c. The segments 10, 15, 20, and 25 are sealed from each other and have independent air-lines 5a-5c attached to segments 15, 20 and 25 respectively to provide for redundancies. The upper-most segment 10 is not only sealed from the other segments 15 through 25 but is also sealed from fluid contact with any surrounding water, having no open port. The lower segments 15, 20, and 25 have open ports 30a-30c to compensate for the high pressures the lower segments see at depth. Without an open port, the deeper a segment is submerged, the greater its wall thickness has to be to avoid collapse, reducing the segments buoyancy. With an open port and pressure from above, thin segment walls are available.
FIG. 2 shows the cross-sectional view of FIG. 1 where air-lines 5a-5c are located around stem 32. The conventional stem 32 is sized to have an inner diameter that is larger than the outer diameter of a riser such that the stem 32 is readily received around a riser.
Segments with open ports are commonly called xe2x80x9csoft tanksxe2x80x9d or xe2x80x9cvariable buoyancy tanks.xe2x80x9d Those that are closed are called xe2x80x9chard tanks.xe2x80x9d Although FIG. 1 shows one hard tank and multiple soft tanks, in practice, multiple hard tanks are used at the top and multiple soft tanks are used at the bottom in a given air-can arrangement. It is also noted that it is not necessary that the tanks be connected to one another in a series arrangement where the air-lines pass through the upper tanks to reach the lower tanks as shown in FIG. 1. One alternative tank arrangement is described by Davies in U.S. Pat. No. 5,758,990, incorporated herein by reference, where a stem having an inner diameter larger than the outer diameter of the riser is positioned around the riser and is fastened in position at the wellhead of the riser on the offshore structure; a yoke attached to the stem supports a number of sleeves around the stem; each sleeve receives a variable buoyancy air can; and the sleeves and air cans are provided with a retainer that retains the air cans in the sleeves and transfers the vertical loads of the air cans to the sleeve.
There are problems, however, with the tanks described above. First, in practice, one cannot pump out all the fluid through the open port in a soft can. FIG. 3 illustrates this problem showing a soft tank 45 including air-line 35 for introducing gases 40, typically air, into soft tank 45 and water 55, indicated by hash marks, below the open port 60. There is a level below which the soft can cannot be evacuated due to the conventional placement and design of the open port. Additionally, in practice, the soft tanks see upward and downward motion. When the tank is moved up, during heave, the water level in the soft tank will drop and the air will escape causing the need to pump more air into the soft tank. To avoid this during normal operations, the water level is left above the open port. Thus, not only the volume below the port is lost for buoyancy, but also some volume above the port is lost. Further, when there is pitch due to wave action at the surface and other forces, the water surface level in the soft tank can drop below the open port, causing air to escape. So, the water level is kept even higher than what would be needed without pitch. Again, volume of the tank is lost for buoyancy.
Downward motion can be caused by forces at the surface or other forces. For a xe2x80x9csparxe2x80x9d structure, as described in U.S. Pat. No. 4,702,321, incorporated herein by reference, as the spar moves laterally, the spar is xe2x80x9coffsetxe2x80x9d from its nominal position. The risers pull the tanks lower in the water, causing the water level in the soft tanks to rise, due to the increase in pressure, again causing a decrease in the available volume for buoyancy, at least without pumping more gases into the soft tank or designing for the offset position, leaving an overcapacity in the soft tank when the spar is in the nominal position.
There is a need therefore, to address the above-mentioned problems.
A riser tensioning device according to the invention comprises a first tank having a central axis; a first passage having a diameter less than the inner diameter of the first tank; the first passage providing a fluid contact between the interior of the first tank and the exterior of the first tank; and the first passage having a portion extending outside the first tank at an angle less than 90 degrees from parallel to the central axis. In one embodiment of the first tank, the first passage is attached in fluid communication with the interior of the first tank at the bottom of the first tank. In one embodiment of the first tank, the first passage is attached in fluid communication with the interior of the first tank at the side of the first tank. In still another embodiment of the first tank, a gas line is in fluid contact with the interior of the first tank.
In a particular embodiment of the invention, the riser tensioning device comprises a second tank having a central axis; a stem connected to the first tank; a second passage having a diameter less than the inner diameter of the second tank; the second passage providing a fluid contact between the interior of the second tank and the exterior of the second tank with the water and the second passage having a portion extending outside the second tank at an angle less than 90 degrees from parallel to said central axis. In one embodiment of the second tank, the passage is attached in fluid communication with the interior of the second tank at the bottom of the second tank. In one embodiment of the second tank, the second passage is attached in fluid communication with the interior of the second tank on the side of the second tank. In one embodiment of the second tank, the second tank is attached to the stem. In still another embodiment of the second tank, the second tank is attached to the first tank.
In a particular embodiment of the attached tanks, the first passage is providing a fluid contact between the interior of the first tank and the exterior of the first tank while passing through the second tank. In one embodiment of the attached tanks, the second tank is attached to the first tank by a stem. In one embodiment of the attached tanks, a gas line is in fluid connection with the interior of the second tank. A particular embodiment of the invention includes the gas line in fluid connection with the interior of the second tank where the gas line passes through the first tank.
In still another embodiment of the invention, the first tank comprises an interior surface having a first corrosion resistance and the first passage has an interior surface having a second corrosion resistance where the second corrosion resistance is greater than the first corrosion resistance. In one particular embodiment, the interior surface having a second corrosion resistance is selected from a group consisting essentially of stainless steel, fiber reinforced pipe, or rubber. In one particular embodiment, the interior surface having a second corrosion resistance is selected from a group consisting essentially of rust inhibiting paint, epoxy, electroplated metals, or thermal sprayed aluminum.
A method of manufacturing a riser tensioning device comprising providing a first tank having an interior surface of a first material; connecting to the first tank a fluid passage having an interior surface of a second material in which the second material is more corrosion resistant than the first material. In one embodiment of the method, the second material comprises stainless steel. In one embodiment of the method, the second material comprises fiber reinforced pipe. In one embodiment of the method, the second material comprises rubber hose. In one embodiment of the method, the second material comprises rust inhibiting paint. In one embodiment of the method, the second material comprises epoxy. In one embodiment of the method, the second material comprises electroplated metal. In one embodiment of the method, the second material comprises thermal sprayed aluminum. In another embodiment of the method, a second tank is provided where the connection of the fluid passage to the first tank is made through the second tank.
A method of providing buoyancy to a riser when in water, the method comprising holding a volume of gas in mechanical connection with a riser; providing a fluid passage between the volume of gas and the water; allowing water to move within the passage in response to vertical motion of the riser while resisting a change in the volume of gas as a result of the vertical motion of the riser. In one embodiment of the method, gas is provided to the volume of gas.
A system for providing buoyancy to a riser when in water, the system comprising means for holding a volume of gas in mechanical connection with the riser, means for providing a fluid path between the volume of gas and the water, means for allowing water to move within the fluid path in response to vertical motion of the riser while resisting a change in the volume of gas as a result of the vertical motion of the riser. One embodiment of the system further comprises means for providing gas to said volume of gas. In one embodiment of the system, the means for holding comprises a tank connected to the riser. In one embodiment of the system, the means for providing a fluid path comprises a passage from the gas to the water wherein a cross-sectional area of the passage is less than a cross-sectional area of the means for holding. In one embodiment of the system, the means for allowing water to move within the fluid path comprises a passage having a length greater than an anticipated vertical motion of the riser.